Continuously variable twin-tube shock damper

ABSTRACT

A twin-tube shock damper has a hollow piston rod (5) which extends towards the top of the working cylinder (1), a baseplate (8) which closes off the working cylinder and a disc (7) which is arranged inside the working cylinder between the piston (4) and the baseplate and has one or more passages (16) and a non-return valve (17) which allows upward flow through the passages, a central tube (10), a connection (11) between the hollow piston rod and the space above the piston, a damping valve (21) which operates in one direction of flow and servo-control means having an excitation chamber (18) and an excitable coil (30). A control valve in the form of a hollow piston (25) movable in a control valve cylinder (22) provided with orifices (24) and a hollow shaft (27) guided in a valve shaft guide (26) provided with orifices (28) are located in the excitation chamber. Both the control valve cylinder and the control valve are pushed by spring means (23, 29) in the direction of the damping valve body (17).

The invention relates to a continuously variable twin-tube shock dampercomprising:

an oil reservoir between a working cylinder and an outer tube,

a piston which is movable in the working cylinder and has a passage anda non-return valve which permits only an upward flow through the saidpassage with negligible damping capacity,

a hollow piston rod which extends towards the top of the workingcylinder,

a baseplate which closes off the working cylinder,

a disc which has one or more passages and is arranged inside the workingcylinder between the piston and the baseplate,

a non-return valve which permits only upward flow through said passages,

a central tube which protrudes through the piston into the hollow pistonrod and through the said disc,

a connection between the hollow piston rod and the space above thepiston,

a damping valve which operates in only one direction of flow and isarranged between the bottom of the central tube,

servo-control means in order to control the flow resistance exerted bythe damping valve as a function of electrical signals which are at leastrelated to the movement of bodywork and/or chassis, which servo-controlmeans comprise an excitation chamber as well as a coil positioned in thevicinity of a permanent magnet, which coil can be excited by the saidelectrical signals in order to control the pressure in the excitationchamber below the valve body of the damping valve,

a control valve having the form of a hollow piston which is arranged inthe excitation chamber, wherein the position of the control valve can bechanged by excitation of the coil and the pressure in the excitationchamber is determined, on the one hand, by inflow of fluid via a narrowpassage and, on the other hand, by outflow of fluid through the controlvalve.

A damper of this type is disclosed in WO 96/08950.

The aim of the invention is so to improve the damper disclosed in theabove publication that the movements of the valve body and the controlvalve are separately damped.

According to the invention, to this end the damper according to thepreamble is characterized in

that the hollow piston is movable in a control valve cylinder providedwith orifices,

that the control valve cylinder is movable within the excitationchamber,

that a hollow shaft forms an integral part of said hollow piston and isguided in a valve shaft guide,

that said hollow shaft is provided with orifices in communication withthe hollow space of the hollow shaft and cooperating with the valveshaft guide so that the orifices are covered or uncovered--depending onthe position of the hollow shaft relative to the valve shaft guide,

and that the control valve cylinder and the control valve are pushed byspring means in the direction of the damping valve body.

If both the guide cylinder of the control valve piston and the guide ofthe control valve shaft have play in the radial direction, alow-friction position control of the control valve can be achieved withlow coil forces. An out-of-true position or jamming cannot occur.

The invention will be explained in more detail below with reference tothe figures.

FIG. 1 shows a longitudinal section of a first embodiment.

FIG. 2 shows a longitudinal section of the bottom section of theembodiment according to FIG. 1 on a larger scale.

FIG. 3 shows a longitudinal section of the bottom section of a secondembodiment.

The continuously variable twin-tube shock damper shown in FIG. 1comprises a working cylinder, constructed as inner tube 1, an outer tube2, a reservoir 3 between the working cylinder and the outer tube, apiston 4, which is movable within the working cylinder, a hollow pistonrod 5 with fixing eye 5a, which can be connected to the sprung part of awheel suspension unit, a cover 6 which closes off the working cylinderand the reservoir and forms a guide for the piston rod, a disc or footvalve housing 7 at the bottom of the working cylinder and a baseplate 8with fixing eye 8a at the bottom of the outer tube.

A central tube 10, the bottom end of which protrudes through the footvalve housing 7, extends through the piston 4.

Openings 11 are made in the piston rod just above the piston 4, whichopenings connect the base above the piston to the interior of the pistonrod 5. The top of the central tube 10 is located above the opening 11.

The piston is provided with a valve housing 12, a valve seat 13 and avalve body 14, which is pushed onto the seat 13 by a spring 15. Thecomponents 12 to 15 form an upstream valve.

Orifices 16 are made in the foot valve housing 7, which orifices areprovided at the top of a spring-loaded upstream valve 17.

An excitation chamber 18, which has a cylindrical section in which abeaker-shaped piston 19 is movable, is located below the housing 7. Atthe top, said beaker-shaped piston has a section which has an axialorifice 20. A valve body 21, which forms part of the damping valve, isplaced on the top section of the beaker-shaped piston 19. The seat ofthe damping valve is located at the broadened underside of a tubesection 9 screwed to the bottom of the central tube 10. The uppersurface of the broadened section of said tube section 9 engages on thefoot valve housing 7.

It will be clear that during the outgoing stroke the oil above thepiston 4 is forced through the openings 11 towards the central tube 10.In the vicinity of the foot of said tube 10, the oil flows through thedamping valve 7, 21 to the channels 16 and via the upstream valve 17into the space below the piston. During the ingoing stroke, the oilbeneath the piston 4 will flow through the upstream valve 12, 13, 14 tothe space above the piston 4, which space is becoming larger. An oilvolume which corresponds to the volume of that part of the piston rod 5which penetrates into the working cylinder 1 likewise flows through thecentral tube 10 to the damping valve 7, 21 and after passing throughsaid valve passes via the openings 16 into the reservoir 3. The upstreamvalve 13, 14, 15 of the piston 4 gives rise to negligible damping; thedamping action is produced by the damping valve 7, 21. The magnitude ofthe damping is determined by the servo-control system located below thevalve body 21.

A relatively small cylinder 22 is arranged so that it is movable withlateral play inside the beaker-shaped piston 19 and said small cylinderis pushed by a helical spring 23 towards the bottom surface of the baseof the piston 19. Radial bores 24 are present in the uppermost part ofthe relatively small cylinder 22. A hollow piston 25 is movable insidethe small cylinder 22 by means of a shaft-shaped section 27, which ismovable in a valve shaft guide 26. One or more radial bores 28 have beenmade in the shaft-shaped section 25. Said bores are covered oruncovered--depending on the position of the piston shaft 27 relative tothe guide 26. A helical spring 29 exerts an upward load on the piston25.

A coil 30 is arranged around the outside of the excitation chamber,which coil is a small distance away from a permanent magnet 31 and canbe moved up and down by electrical excitation. The coil 30 is connectedto the shaft section 27 of the hollow piston 25.

Electrical excitation of the coil leads to upward or downward movementthereof, during which movement either the piston 25 is moved downwardsagainst the pretensioning of the spring 29 and, therefore, the controlvalve is pulled open, or, as a result of reversal of the direction ofthe current, the piston is moved together with the pretensioning of thespring 29 in the direction of closure. In the first case (opening of thecontrol valve), the oil pressure below the beaker-shaped piston 19becomes lower, with the result that the damping valve opens more rapidlyand damps to a lesser extent. In the second case (closing of the controlvalve), the oil pressure below the beaker-shaped piston 19 becomeshigher as a result of which the damping valve opens with more difficultyand damps to a greater extent.

In fact, the pressure in the excitation chamber 18 is controlled by aforce equilibrium between a force which is the consequence of oilpressure on a surface the magnitude of the shaft section 27 of thecontrol valve, a force generated by the spring tensioning of the spring29 and a force which is the consequence of the electromagnetic force ofthe coil 30. Triggering of the coil 30 is effected from an electroniccontrol unit via cable 32, conductor pins 33, line 34, laminated springs35 and resilient tongues 36. The laminated springs 35 have a negligibleinfluence on the vertical position of the coil 30.

The shock damper also has a direction sensor 37 which, by means of amechanical contact, or by measurement of a movement by means of coils inaccordance with the Hall effect, or piezo-electrically, converts thepressure difference between the bottom of the cylinder and the reservoir3 into a signal which indicates the direction of movement of the pistonrod. Via resilient pins 38 and conductor pins 39, the direction sensor37 connects a cable 37 to the electronic control unit (ECU).

The pressure in the excitation chamber 18 is determined by the inflow ofliquid via the passage 20 and the outflow of liquid via the radial bores28.

As a consequence of the abovementioned force equilibrium, there is anindirect feedback relationship between the current through the coil 30and the damping force on the piston rod 5.

When the coil 30 is not excited (fail-safe position), the pretensioningof the helical spring 29 and the cross-sectional surface area of theshaft 27 of the control valve determine the pressure in a highlycontrolled manner, liquid issuing from the excitation chamber. Saidpressure can be so chosen that the damping force constitutes an averagebetween the maximum and minimum damping force. The fail-safe position(without control) is automatically ensured.

The relatively small cylinder 22 and the valve shaft guide 26 have someradial play. Consequently, low-friction position control of the controlvalve 25, 26 can be achieved with low force from the exciter coil 30.

The movements of the piston 19 in the chamber 18 cause a certain amountof damper fluid to move, with the entire surface of the piston asdisplacement surface and with a resistance to flow, when control valve25, 27 is stationary, caused by the radial bores 24 in the smallcylinder 22. Conversely, any movement of the control valve 25, 27 whenthe piston 19 is stationary is damped in the openings 24 in the smallcylinder 22. Consequently, there are two damping characteristics. Thepiston 19 with piston helical spring 22, on the one hand, and the valveshaft 27 with coil 30 and helical spring 29, on the other hand, give twodifferent inherent frequencies and two different damping values.

The invention according to FIG. 3 differs from that according to FIGS. 1and 2 in respect of the design of the damping valve and in that thebeaker-shaped piston 19 has been dispensed with, as has the tube section9 screwed into the bottom end of the central tube 10.

The valve body 21 is constructed as an assembly of a number of thinspring steel discs (membranes), which are held together by a centringpin 21a with spindle retaining ring. The axial passage 20 is made in thecentring pin. The discs are fixed with some play at their circumferencebetween the housing 7 and the wall of the excitation chamber 18. Sealingis effected by means of an O-ring or the like in a groove in the wall ofthe excitation chamber. In this case as well, the damping liquidsupplied via the central tube is fed via orifices 41, 16 in the housing7 into the reservoir 3, at least when the valve body 21 has moved freeof the seat beneath the housing.

It will be clear that the servo-intensifying effect is achieved with asmall surface area and a high pressure (up to 100 bar) above the valvebody 21 and a large surface area and a low pressure (down to 4 bar)below the valve body 21. The functions which in the embodiment accordingto FIGS. 1 and 2 are fulfilled by the beaker-shaped piston 19 are takenover by the membrane valve body 21 in the embodiment according to FIG.3. The side of the membrane package 21 which faces towards theexcitation chamber 18 is subjected to the pressure which prevails insaid chamber and which is controlled by the valve 25, 26 and the coil30. The contact pressure of the package 21 against the seat at thebottom of the housing 7 can be controlled and consequently theresistance to flow of the liquid which flows from the openings 41 to theopenings 16 proportionally to the electrical control current through thecoil 30.

We claim:
 1. Continuously variable twin-tube shock damper comprising:anoil reservoir (3) between a working cylinder (1) and an outer tube (2),a piston (4) which is movable in the working cylinder (1) and has apassage and a non-return valve (12, 13, 14) which permits only an upwardflow through the said passage with negligible damping capacity, a hollowpiston rod (5) which extends towards the top of the working cylinder, abaseplate (8) which closes off the working cylinder, a disc (7) whichhas one or more passages (16; 41) and is arranged inside the workingcylinder between the piston and the baseplate, a non-return valve (17)which permits only upward flow through said disc passages, a centraltube (10) which protrudes through the piston into the hollow piston rodand through the said disc (7), a connection (11) between the hollowpiston rod and the space above the piston, a damping valve (9,21;7,21)which operates in only one direction of flow and is arranged adjacentthe bottom of the central tube, servo-control means to control the flowresistance exerted by the damping valve as a function of electricalsignals which are at least related to the movement of a bodywork and/orchassis, which servo-control means comprise an excitation chamber (18)as well as a coil (30) positioned in the vicinity of a permanent magnet(31), which coil (30) can be excited by the said electrical signals inorder to control the pressure in the excitation chamber below a valvebody (21) of the damping valve (9,21;7,21), a control valve (25,27)having the form of a hollow piston (25) which is arranged in theexcitation chamber (24), wherein the position of the control valve(25,27) can be changed by excitation of the coil (30) and the pressurein the excitation chamber (24) is determined, on the one hand, by inflowof fluid via a narrow passage (20) and, on the other hand, by outflow offluid through the control valve (25,27), characterised in, that thehollow piston (25) is movable in a control valve cylinder (22) providedwith orifices (24), that the control valve cylinder (22) is movablewithin the excitation chamber (18) that a hollow shaft (27) forms anintegral part of said hollow piston (25) and is guided in a valve shaftguide (26), that said hollow shaft (27) is provided with orifices (28)in communication with the hollow space of the hollow shaft (27) andcooperating with the valve shaft guide (26) so that the orifices arecovered or uncovered--depending on the position of the hollow shaft (27)relative to the valve shaft guide (26), and that the control valvecylinder (22) and the control valve (25,27) are pushed by spring means(23, 29) in the direction of the damping valve body (21).
 2. Shockdamper according to claim 1, characterised in that the valve body (21)of the damping valve is combined with a piston (19) which has a liquidpassage, which piston is movable up and down inside a cylindrical wallin the excitation chamber (18).
 3. Shock damper according to claim 1,characterised in that the valve body (21) of the damping valve iscomposed of a number of spring steel discs held together by a centeringpin (42) provided with a liquid passage (20).
 4. Shock damper accordingto claim 1, characterised in that in the case of a non-excited coil,pretensioning of the spring means (29) on the control valve piston (25,27) and the cross-sectional surface area of hollow shaft (27) aredetermined in such a way that the damping force constitutes a mean ofthe maximum and minimum achievable damping force at the top of thepiston rod.
 5. Shock damper according to claim 1, characterised in thata direction sensor (37) is fitted in the shock damper, which directionsensor converts the pressure difference between the bottom of theworking cylinder (1) and the reservoir (3) into a signal and transmitssaid signal to an electronic control unit which transmits the electricalsignals to the coil (30).
 6. Shock damper according to claim 1,characterised in that both the control valve guide cylinder (22) and thevalve shaft guide (26) have play in the radial direction.